Non-orthogonal multiple access for cellular-connected unmanned aerial vehicles

Abstract

Cellular-connected unmanned aerial vehicles (UAVs) have been introduced for 5th Generation (5G) and beyond cellular networks to enable various UAVs’ operations which require real-time and ubiquitous connectivity. Existing solutions are relying on orthogonal multiple access (OMA) to support existing terrestrial users (TUs) and UAVs as new aerial users (AUs). However, OMA is unable to provide an efficient network performance because each orthogonal resource block can only be utilised by a single user. To address this limitation, non-orthogonal multiple access (NOMA) can be employed. NOMA enables AUs and TUs to share the same orthogonal resource block. By leveraging their downlink asymmetry, NOMA could efficiently serve the AUs and TUs. Nevertheless, concurrently serving the AUs and TUs in cellular networks introduces new challenges. Specifically, reverse successive interference cancellation (SIC) policy and inappropriate NOMA power allocation might occur if the AUs are moving in three dimensional space and perfect channel state information (CSI) is unavailable. These issues will result in spectral inefficiency and unreliable communications. Due to high altitude, AUs also suffer strong inter-cell interference (ICI) that causes the pairing of AUs and TUs in NOMAto be inefficient. Therefore, this thesis investigates the performance of NOMA which concurrently serves a mobile AU and a TU in the absence of perfect CSI. Results show that pairing a mobile AU and a TU is more beneficial than pairing TUs only. Furthermore, NOMA provides up to 99% rate of improvement and lower outage probability as compared to OMA. Performance analysis for AUs and TUs in multi-cell networks is also carried out by using stochastic geometry. The analysis highlights the effects of different network parameters and reveals that the network performance can be affected by user association, receiving antenna configuration and ICI mitigation technique. This thesis proposes and provides an important insight about an efficient combination of user association, transmitting and receiving strategies known as aerial-terrestrial network NOMA. The proposed scheme outperforms existing schemes up to 91% in terms of sum-rate and its analytical outage probability can be as low as the order of 10-17. This thesis concludes that NOMA can efficiently serve the AUs and TUs in downlink cellular networks.